A so-called PS plate (Presensitized Printing Plate), which comprises an aluminum plate having a photosensitive composition coated thereon in a layer structure, has been known as a type of lithographic plate. A surface of the aluminum plate is roughened using various kinds of methods. After the surface roughening process, the aluminum plate is etched with an acidic or alkaline solution, further subjected to an anodic oxidation processing, and optionally receives such a treatment as to render the surface thereof hydrophilic. The thus processed aluminum plate is used as the support of a lithographic plate. On this support is provided a photosensitive layer to prepare a presensitized or PS plate. The PS plate is subjected to electromagnetic radiation exposure, development, correction, gumming-up and other process steps to be converted into a printing plate. The printing plate is set up in a press, and printing is performed.
The following methods for processing the surface of the aluminum plate have been conventionally known mechanical surface-roughening methods, such as ball graining, wire graining, brush graining, liquid honing, etc.; electrochemical methods, referred to as electrolytic graining methods; chemical methods; and combinations of two or more of such surface-roughening methods. However, these conventional methods have their respective, unsolved problems as described below.
In the case of the ball graining method, an especially high level of skill is required in selecting the kind, quality, and size of balls to be used, controlling the quantity of water to be used during graining, choosing the graining time, choosing the graining conditions by evaluating the condition of the finished-up surface such as a grain shape, uniformity of grains, etc., and so on. Furthermore, the productivity is remarkably low, particularly because the batch method is employed therein.
According to the wire graining method, the grain formed on the surface of an aluminum plate lacks uniformity. On the other hand, when the brush graining method is employed, problems arise such as that great roughness cannot be obtained at the grained surface, and the degree of roughness tends to vary with the graining time because of the abrasion loss of the graining brush used. In addition, the aluminum surface is scratched complexly through strong rubbing of the brush end and abrasives thereagainst to result in formation of a considerable number of sharp bar-like projections. It is attributable to these projection that some of the photosensitive layer remains in the areas intended to be removed upon development of the PS plate, to thus cause the staining of the plate surface, and aluminum plates are liable to receive scratches at their surfaces through casual rubbing of one processed face (roughened face) with another processed face upon handling thereof.
In the case of liquid honing, a slurry obtained by dispersing fine abrasive powders into a liquid is blown acceleratively against an aluminum plate by compressed air or the like. Therefore, the fine abrasive powders tend to stick into the aluminum plate to form burs. Further, the impact power of the slurry on the aluminum plate is too weak to roughen the plate surface to a sufficient extent in some cases. Furthermore, this method has another problem that the nozzle for projecting the slurry tends to be worth away in a short period of time due to the accelerative spurt of the slurry.
In the electrochemical surface-roughening method, the regulation of electrolysis conditions must be carried out with high precision in order to obtain regular graining at the roughened surface, the consumption of electric power is significant, and the waste solution containing Al ions, which remains and accumulates in the electrolytic solution, is disposed of at great cost.
In the case of chemical surface-roughening method, it takes a long time to achieve the treatment, so this method is not well suited to mass production. Further, a great cost is required for the waste disposal, as in the case of the above-described method. In this respect also, it is not well suited for mass production.
As surface-roughening methods which solve part of the above-described points to acquire suitability for mass production, there are disclosed a combined surface-roughening method, in which a brush or wire graining process is combined with an electrolytic graining process (e.g., in Japanese Patent Application (OPI) No. 63902/79 (the term "OPI" as used herein means an "unexamined published application")), and a combined surface-roughening method in which a liquid honing process is combined with an electrolytic graining process (e.g., in Japanese Patent Application (OPI) No. 19593/85). However, even these methods still have problems, such as that they cannot prevent stains from generating during printing, and are insufficient in press life.
In addition, there are disclosed the methods of graining with a slurry accelerated using high pressure water (in, e.g., Japanese Patent Application (OPI) No. 214697/84 corresponding to U.S. Pat. No. 4,613,413) as those which aim at the solution of the foregoing points, that is to say, to have suitability for mass production, to prevent stains from generating upon printing, and to ensure excellent printing press life.
However, the above-described method, in which the graining is carried out using slurry accelerated with high pressure water, still has a problem that it is necessary to circulate large quantities of slurry water and high pressure water.